Short-time-limit switch



G A BURNHAM SHORT TIME LIMIT SWITCH Filed May, I?, 1917 Apr. 3, 1923.

www w Patented Apr. 3, 1923.

UNITED STATES PATENT OFFICE.

GEORGE A. BUBNHAM, 0F SAUGUS, MASSACHUSETTS, ASSIGNOR T0 SEARS B. GONDIT, JR., 0F BBOOKLINE, MASSACHUSETTS.

- SHORT-TIME-LIMIT SWITCH.

Application led Hay 17, 1917. Serial N0. 189,227.

To all 'wh-0m z't may concern:

Be it known that I, GFRGE A. BURNHAM, a citizen of the United States, residing at Saugus, in the county of Eex and State of Massachusetts, have invented an Improvement in Short-Time-Limit Switches, of which the following is a specification.

My invention relates to that class 'of electric switches employed for the control and protection of induction motors, and has particular reference to that type'adapted to be actuated to interrupt the circuit upon the occurrence of an overload or upon the temporary cessation of energy in the line.

In the operation of induction motors of small powers it is customary to 'dispense with the usual starting transformer or compensator and to connect the motor directly to the line and to provide trip coils in circuit with the motor to cause the switch to open upon a harmful overload.

In connection with the trip coils, it is customary and highly desirable to employ some form of time limit element, to retard the action of the coils in causing the opening of the switch. Momentary overloads, even though quite severe, will not injure the motor and; it is an annoyance and a cause of a lo in production that is of importanceiin the commercial use of the motors, to be obliged to restart the motor after a momentary harmless overload. Con.

sequently, it is customary to employ a long time element in connection with the tri coils of the switch, or an element that wi l retard the action of the trip coils in causing the opening of the switch fora comparatively long time under la heavy overload.1

With such a long time element associated with the trip coils of the switch, Ythe current;

required for the starting 'of the motor d oes not energize the coils for a sufficient period of time to cause the opening of the switch.

It is also customary to associate with the motor switch what is commonly known as `a novoltage release, the urpose of which is to cause the opening o the switch upon cessation of motor voltage from some cause not brought about by the pening of the motor controlling switch. he no-voltage release consists of a coil connected across the line, the armature of which is adapted to be released upon de-energization of the coil, and, by its release, to cause the opening of the switch.

The no-voltage release serves tov prevent the motor from being unknowingly restarted upon re-ener ization of the line and thus prevents possib e damage to the machine to which it is connected or to the material being worked upon by the machine.

An object of my invention is to incorporate, in a switch for the control of motors, a single electrically-operated device that will serve to protect the motor against a dangerous overload; that will prevent the restarting of the motor upon its stop ing due to a temporary de-energization of t e motor not caused by the opening of the motor control switch; and that will permit the motor to be started without operating to open the switch.

A further ob'ect of my invention is the novel method o operating induction motors by which the motor is disconnected from the circuit due to failure of the electric supply.

When an induction motor has been stopped because of an accidental loss of energy in the line, and not because of the opening of the motor control switch, and this energy is subsequently supplied to the line, with the usual type of motor control occasioned by the .re-energization of the lines with the motor connected across the line to cause the effective energization of my electrically operated protective devi to cause the opening of the switch before the motor has been sufficiently energized to rotate and thus incorporate, in one device, the function of an overload trip coil and the function of a no-voltage release, while also retaining all the features of a time limit opening means, and the means by which I accomplish this desirable and heretofore unattained resultvis in the peculiar design of the time limit element associated with the overload trip coil. 1

A further object is generally to improve the construction and operation of an electric switch.

Certain features of this invention are disclosed in my Patent No. 1,399,460, dated December 6, 1921. I

Fig. 1 is a transverse section of a switch embodying my invention.

Fig. 2 is a diagram of the circuit connections.

Fig. 3 is a typical time-current curve of an induction motor.

Fig. 4 is a section along line 4 4 of Fig. 1.

Fig. 5 is a' detail of the operating mecha` nism illustrating the relative position of the parts thereof in the open position of the switch.

Fig. 6 is a detail similar to Fig. 5 and illustrating the position ot the mechanism with the switch in starting position.

Fig. 7 is a detail similar to Fig. 5 and illustrating the position of the mechanism with the switch in running position.

As here shown, an embodiment of my invention is incorporated in the double-throw switch shown in Fig. 1 and described in l greater detail and claimed in my Patent No.

1,399,460, dated December 6, 1921, in which the base 10 supports the two sets of contacts 11 and 12 between which are disposed the movable set of contacts 13 supported by liexible spring conductors 14, the position of which is controlled by a rod 15 connected yby an insulated block 16 to the spring conductors 14 and to an arm of a bell crank 17. A second arm of the bell crank 17 is connectedto the operating handle 18 by a link 19. An arm 20, associated with handle 18 carries a pin 21 that operates in a slot 22, having a recess 23 in link 19 and said pin is under the control of latch 24 pivoted at 248L to said link 19. This form of connection between the operating handle 18 and the bell crank lever 17 forms means whereby the movable switch contacts may bemoved into open position independently of the operating handle upon the operation of the tripping mechanism, as is more fully described in the above mentioned patent.

In the neutral or switch-open position of the mechanism, Figs. 1 and 5, the pin 21 of the operating handle 18 is or may be in some mid-position in the slot 22 in the link 19 and is free from positive connection with said link and consequently with the operating mechanism. When the handle is operated to move the switch to starting position, the pin 21 is moved to the left, Fig. 5, in the slot 22 and tits within the recess 23 of said slot, the link 19 dropping downward and the latch 24 being moved away from the recess by said pin, for this purpose. The pin 21 is then held releasingly by the latch in said recess 'and in operative engagement with said link and further movement of the pin serves to move said link, and the remainder of the switch mechanism associated therewith, to the starting position of the switch. A movement of the operating handle and the pin 21 in the reverse direction effects the movement of the operating mechanism into a running position indicated in Fig. 7. Upon an overload, the pin 35 is actuated by the trip coils 26 to raise the latch 24 and move it from beneath the pin 21, and also raise the link 19 to clear said pin, and the switch consequently moves to open or neutral Figs. 1 and 5.

Depending from base 10 is a casing 25 adapted to contain oil. Depending also from base 10, and arranged within the casing 25, are two trip coils 26 with their corcs 27 slidable in tubes 28 and normally maintained against the adjustable stops 29, the position of which serves to determine the value of overload at which the cores will be effectively attracted to operate to cause the opening of the switch. Retarding disks 30 are secured to the lower ends of cores 27 by means of screws 31, the tubes 28 being provided with slots 32 to provide for the free movement of cores 27 with the attached disks 30, and certain of screws 31 are made with extensions 33 to engage with the enlarged portion34 of a latch operating pin 35 which projects through base 10- and terminates in close proximity with the under side oflatch 23.

Disks 30, in combination with the oil in casing 25, form a time limit element for trip coils 26, the retarding action of which can be varied by varying the shape and dimensions of the disks 30.

A diagram of connections between a three- `phase line, switch and motoris shown in Fig. 2, in which the line wires 36 are connected to the movable switch members 13 disposed between the starting contacts 11 and running contact 12. Starting con@ tacts 11 are directly connected to motor M by'conductors 37 while two of running contacts 12 are connected to motor M through the trip coils 26.

In the ope'ration of the switch, the contacts 13 are first brought into engagement with starting contacts 11. A` large excess of current immediately flows through the motor as indicated at a in Fig. 3. This current continues to flow, with decreasing strength as the motor increases in speed until a point b on the time current curve is reached, at which point both the speed and current have reached normal values. At this point, the movable contacts are shifted from starting to running position. During the time the movable contacts 13 are being' moved from stationary contacts' 11 to 12, there is no current flowing through the motor, this drop in current being indicated by point c in the curve. Due to this momentary loss of energy. Vthe motor speed decreases and when the connection with the line is again position as indicated in made through the running side of the switch, a momentary second rush of current occurs as indicated by d on the curve, during which rush of current the speed is again brought up to normal and the current drops to some normal value e.

' This second current peak exists for but a short time, say in the neighborhood of two one-hundredths of a second, compared with the duration of flow of the excessive starting current and its value is rarely more than twice normal current flow, while the starting current reaches a value of from five to six times normal current iow.

It is evident that the time limit device must so restrain the action of the core of the trip coil that the switch will not be caused to move to open position by the excess current flow occasioned by the movement of the switch from its starting to running position as otherwise the switch could never be maintained in running position long enough for the motor to attain its normal speed, and it is further evident, to prevent the motor from accidental starting, after it has stopped from loss of line energy, with the switch in running position, or'in othervwords to perform the functions of a no-voltage release, ,that the time limit element must have such a limited delayed action that the-switch will be caused to move to open position, upon the re-energization of the line and the consequent excessive current fiow through the motor, before the motor has had time to appreciably rotate.

With these conditions fulfilled, the usual overload trip coil, associated with my invention,'wi1l perform its own functions as well as those of the 11o-voltage release, and alsogive .satisfactory delayed action upon overloads.

In the operation of the overload coils having the requisite time limit element, upon the cle-energization of the line, the motor ceases to revolve and the Switch remains in closed running position, during the period of line de-energization. Upon the lre-energization of the line, a heavy starting-current immediately fiows through the motor and `through the overload coils. Due to the short time element, the trip coils are permitted to v operate to open the switch before t-he motor has had time to appreoiably revolve. The motor is thereby prevented from operation, and motor and line are protected against injury due to the heavy current flow permitted by a previous cessation of line energzation, upon the beginning of the' heavy current flow, just as effectively and safely as though the subsequent heavy current flow had been prevented by the opening of the line immediately upon the cessation of line energization.

I claim 1. An induction motor starting and controlling device including an electric switch having starting and running positions, an operating handle movable between starting and running positions and arranged to be held in running position to hold the switch in running. position and operating mechanism releasingly connect-ing the handle with the switch, and overload tripping means arranged to break the connection between the operating mechanism and handle and effect the opening of the switch from running position independently of movement of the handle from running position, said switch arranged to be moved to starting position to connect the motor directly to the line for starting and exclude said tripping means, and arranged to be moved from starting to running position and momentarily interrupt the motor circuit and reconnect the motor to the line through said tripping means, said tripping means having retarding means associated lwith it arranged to hold the tripping means against operative movement due to the momentary overload resulting when the motor vcircuit is re-established on the running side of the switch and to operate substantially instantlyrto trip the switch upon an overload having characteristics substantially exceeding those of the aforesaid momentary overload.

2. An induction motor starting and controlling device including an electric switch having starting and running positions, an operating handle movable between starting and running positions and arranged to be held in running position to hold the switch in running positionyand operating mechanism releasingly connecting the handle with the switch, and overload tripping means associated with' the running position of the switch arranged to break the connection between the operating mechanism and handle and effect the movement of the switch`to open position independently of movement of the handle from running position, said tripping means having retarding means associated with it arranged to hold the tripping means against effective operation due to the momentary overload resulting when the switch is thrown from starting to running position, and effect its operation to trip the switch upon overloads having characteristics substantially exceeding those of the aforesaid momentary overload.

3. An electric switch comprising a casing adapted to contain oil, stationary and movable switch members immersed in -the oil in said casing, operating mechanism arranged releasingly Ato hold the switch closed, and overload switch-tripping means having fluidgoverned retarding means associated with it also contained in said casing and immersed in the oil therein.

4. An electric switch comprising a casing adapted to contain oil, stationary and movable switch 'members immersed in the oil in name to this specification, inthe presence of said casing, operating mechanismv arranged two subscribing Witnesses. releasingly to hold the .switch closed, and

an overload trip coil disposed Within said. GEORGE Af BURNHAM'. .5 casing having fluid-governed retaroling Witnesses:

means immersed in the oil in said casing. T. T. GREENWOOD,

In testimony whereof, I have signed my H. B. DAVIS. 

